There exist a variety of work machines, for example, hydraulic excavators, wheel loaders, off-highway vehicles, mining machines and other heavy construction vehicles or machines which are used to perform a variety of tasks. In order to achieve this, work machines require a power source, such as, a diesel engine, a gasoline engine, a natural gas engine, a turbine engine or any other type of power source that provides the required power. Such work machines often further include various hydraulically-powered implements or hydraulic drive motors.
Generally, work machines include a pump operatively coupled to the power source for producing a flow of pressurized hydraulic fluid to power implements or drive motors of the machine. In many work machines, the pump is of a variable displacement types. Control systems of these machines adjust the fluid volume displaced by the pump each cycle based on various operating conditions or requirements. For example, the control systems may increase the displacement of the hydraulic pump in response to increased power needs of various work machine implements. Increasing, the displacement of the pump also increases the load the pump places on the power source, which may adversely affect operation of the power source. In some circumstances, if a variable-displacement pump is operated at a relatively high displacement, the power requirements of the pump may exceed the power capacity of the power source.
A variety of rotary pumps with displacement control are known, for example the device as described in U.S. Pat. No. 3,727,521 which discloses an axial piston pump including a rotary cylinder block with reciprocal pistons controlled by an adjustable swash plate for varying displacement as the cylinder block rotates against a valve plate, together with a control port in the valve plate for supplying fluid under pressure to the pumping pistons for controlling the position of the swash plate and therefore the displacement through the medium of the pumping pistons rather than separate control means.
Further prior art such as GB patent 521887 discloses a hydraulic control system having a variable delivery pump which supplies a motor through a throttle valve, means are provided for maintaining the pressure drop across the throttle which means is controlled by the pressure between the pump and the throttle and between the throttle and the motor. A variable delivery pump is used to drive a motor through a system which includes the control valve and a throttle. The pipe lines to and from the throttle are connected to the branches which pass to a control cylinder that is used to vary the delivery of the pump. When the pressure difference across the throttle opening varies from the predetermined one, the result of difference of pressure in the lines causes movement of the controlled piston which regulates the pump until the datum difference is re-established.
Known valve controlled pumps may control the flow of fluid use a check valve rather than a valve plate. However, these devices usually do not seek to provide a variable fluid flow as a mechanism to achieve this is too complex and, so can be unreliable. Digital Displacement Pump® or DDP technology may use computer driven valves rather than a mechanical approach for flow control. However, using this technology in valve controlled pumps requires a large overhead to switch the valves quickly and in the correct synchronicity with the angle of the shaft and actuation of the valves.